A p-n-p-n diode known as Shockley diode or thyristor, is a solid-state semiconductor device similar to two-terminal p-n diode, with an extra terminal which is used to turn it on. Once turned on, diode (p-n-p-n diode or n-p-n-p diode) will remain on conducting state as long as there is a significant current flowing through it. If the current falls to zero, the device switches off. Diode has four layers, with each layer consisting of an alternately p-type or n-type material, for example p-n-p-n and n-p-n-p. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to one of the middle layers. The operation of a diode can be understood in terms of a pair of tightly coupled transistors, arranged to cause the self-latching action.
Diodes are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to automatically switch off; referred to as ‘zero cross operation’. The device can also be said to be in synchronous operation as, once the device is open, it conducts in phase with the voltage applied over its anode to cathode junction. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from anode to cathode, and so is asymmetrical in nature.
These properties are used control the desired load regulation by adjusting the frequency of the trigger signal at the gate. The load regulation possible is broad as semiconductor based devices are capable of switching at extremely high speeds over extremely large numbers of switching cycles.
In FIG. 1A, the schematic of diode is illustrated. It consists of four terminals, such that the anode 111 is connected to power supply or regulating node, the base 112 of p-n-p transistor 115 serves as the collector 112 of n-p-n transistor 114, the collector 113 of p-n-p transistor 115 serves as the base of n-p-n transistor 114 which is controlled by the voltage controller 116. In order to turn on diode and hold the state of turn-on, the voltage controller should raise the voltage from ground level to VF (forward bias, 0.6 v˜0.8 v for silicon). And the voltage controller 116 should supply the current 117, referred as the base current, which current depends on the characteristic of transistor 114 and 115. Once the base current 117 establishes the forward bias (VF), the collector 112 of n-p-n transistor 114 holds the current path 119 from the base of p-n-p transistor 115. After then, p-n-p transistor 115 is turned on because the base 112 has forward bias from the emitter 111. This makes the current path 118 which can keep the turn-on state. This is the holding state as long as the base has not so much leakage to drive the base voltage under forward bias (VF) even though the voltage controller 116 is open. To turn off diode, the voltage controller 116 should lower the voltage of the base of n-p-n transistor 114 under forward bias. To do so, the voltage controller 116 should flow more current than the current path 118.
Diode can hold the states of turn-on or turn-off, but it has very high holding current to store ‘on’ state. There are many prior arts to apply diode to a capacitive memory devices, such as, “High density planar SRAM cell using bipolar latch-up and gated diode breakdown”, U.S. Pat. No. 6,104,045, and “Thyristor-type memory device” U.S. Pat. No. 6,967,358 and “Semiconductor capacitively-coupled negative differential resistance device and its applications in high-density high-speed memories and in power switches”, U.S. Pat. No. 6,229,161. These prior arts require very high holding current and multiple internal voltage generators, in order to use a diode itself as a holding device which becomes a memory cell. And there is another report, “A novel capacitor-less DRAM cell Thin Capacitively-Coupled Thyristor (TCCT)”, 2005 IEEE Electron Devices Meeting (IEDM) Tech. Dig. pp. 311. This approach requires very deep negative voltage in order to write data because the inversion layer of the gate is attached to the drain/source region (or emitter/collector), the gate can swing only ground to deep negative voltage (−1.5V) to avoid the leakage path to the drain, which needs negative pump circuit or external negative voltage. And high current flowing eventually raises operating temperature by “Joule heating”, which produces more junction leakage and gate leakage. Consequently, the data stored in diode can be lost quickly by those leakages, which means that it is difficult to use diode itself as a memory device.
Another approach is that diode replaces the MOS access device as a switching element, not holding device. However diode can not easily replace the MOS device as an access device because it has unidirectional current control characteristic and internal feedback loop. Now the present invention devotes to replace MOS transistor with a diode as an access device and a control methodology has been invented to control the diode for memory operation. Diode can work for the memory devices as a switching element, not a storage element. Furthermore, diode can replace sense amplifier as well, such that diode output generates information “on” or “off” which is digital value. It gives us as many as advantages to design and fabricate it on the wafer.
Separately a capacitor is still required to store data as the conventional memories such as DRAM, but now there is no need of high capacitance to drive bit line directly. Instead, the capacitor drives only one of diode terminals which has very little capacitance, and the capacitor communicates to bit line (or data line) indirectly, while diode directly communicates to bit line. As a result, diode serves as a sense amplifier to detect whether the storage node voltage is forward bias or not. This is different control method from the conventional DRAM, where the gate of MOS transistor is connected to word line and turns on and off, but the load of gate is only gate and routing capacitance, while the storage capacitor drives the very heavy bit line directly, which means that word line loading is very light. Conversely, using diode as an access device gives bit line loading to word line through diode, which makes word line loading very heavy, but it is controllable to design with strong driver or segmentation for word line. Even though word line loading is high, it is desirable to configure a memory array because word line driver is stronger than the storage capacitor. In the conventional DRAM, the weak storage capacitor directly drives bit line, which needs time to redistribute charge from the capacitor to bit line. The stored charge was lost during read cycle by the charge redistribution, which is referred as destructive read. Memory read cycle was very slow because each read cycle requires additional restore procedure.
Furthermore, word line should be higher level than that of bit line to reduce threshold voltage drop by the access NMOS transistor. In case of PMOS access transistor, word line should be negative during read or restore. Those consume high switching current and pumping current. And MOS access transistor has subthreshold leakage current which is tricky and hard to reduce. In order to reduce subthreshold leakage current, the body of the MOS transistor is applied negative voltage for NMOS transistor, but the internal negative voltage generator consumes current and needs to be adjusted for the optimum voltage level for the use. And one more undesirable effect is the parasitic bipolar transistor in the bottom side of the MOS transistor which should be suppressed by applying the negative voltage to the body. The slight forward bias can remove the stored charge to the body.
Applying a diode as an access device, memory array design has a lot of freedom escaping from the MOS device. Additionally, the capacitor can be reduced, and any of capacitor can be used for storing data. Depending on the capacitor material, the retention time and the write time are different. For example, DRAM uses ordinary dielectric capacitor, such as silicon dioxide, silicon nitride, Ta2O5, TiO2, Al2O3, TiN/HfO2/TiN(TIT), and Ru/Insulator/TiN(RIT), which can store data in the range of 300 ms to 1 sec. It is called volatile memory. Alternatively, ferroelectric capacitor can be used as a storage capacitor, such as lead zirconate titanate (PZT), lead lanthanum zirconium titanate (PLZT), barium strontium titanate (BST), and strontium bismuth tantalate (SBT), as shown in the prior art, “Ferroelectric Random-Access Memory”, U.S. Pat. No. 5,600,587. In the present invention, ferroelectric capacitor can be used as a volatile memory because the stored charges are gradually discharged after the electric field is off. Moreover read operation is different from FRAM (Ferroelectric Random Access Memory), such that plate line is not moving when read in the present invention, while plate line moves in FRAM operation in order to measure the polarized charges in the capacitor. Thus the memory operation is still volatile mode, but retention time would be increased as long as high dielectric constant material is used.
In the prior art, the storage capacitor directly drives the heavily loaded bit line through MOS transistor, which means that read operation depends on the charging time of the weak storage capacitor to heavy bit line parasitic capacitor. Also turn-on resistance of the MOS transistor is higher than that of diode. Thus discharging is slow with high RC time constant. These are major disadvantages of DRAM operation. Moreover read operation is destructive which requires the write-back operation. In consequence, DRAM operation is very slow.
Still, there is a need in the art for a memory circuit and cell for random access memory devices, which realize low power, high density and simple structure to fabricate on the wafer. In the conventional MOS access transistor as shown in FIG. 1B, there is a parasitic n-p-n bipolar transistor wherein the body 134 serves as the base, source/drain serve as the emitter/collector. During read and write cycle, the base (body) 134 is at ground (or negative) to prevent bipolar effect. The parasitic bipolar transistor is not wanted device in the conventional memories which is usually turned off, but now adding one more terminal to the parasitic bipolar transistor in the conventional memory, a p-n-p-n diode (or n-p-n-p) can serve as four-terminal diode access device for the next generation memory devices with good performance and simple structure. Separately, storage capacitor is required to store data, but there is no need of high capacitance because the storage capacitor only drives the base of bipolar transistor while strong diode drives heavy bit line.